Rate sensors such as vibrating structure gyroscopes are known which have been constructed using a variety of different structures. These include beams, tuning forks, cylinders, hemispherical shells and rings. A common feature in all of these designs is that they maintain a resonant carrier mode oscillation. This provides the linear momentum which produces a Coriolis force when the gyro is rotated around the appropriate axis.
A known balanced tuning fork configuration, as shown in the schematic form in the accompanying FIG. 1, is perhaps the most common structural type. For this mechanisation the fork tines 1 are set into motion 180.degree. out of phase, in the plane of the fork structure. The drive is tuned to the resonant frequency of the mode to maximise the amplitude of motion for any given drive level. Accurate information about the material mechanical properties and control of the dimensional tolerances is necessary to balance the frequencies of the tines 1. This ensures that there is no net force or torque around the centre of mass and reduces sensitivity to linear accelerations. An angular rate, .omega., applied around the axis of a stem 2 at the fork will generate Coriolis forces in the axis orthogonal to the carrier vibration and rotation axes. The tines of the fork will exhibit an anti-phase vibration, as shown in FIG. 1, at the carrier mode frequency. The amplitude of this vibration will be proportional to the applied rotation rate 3.
It has been proposed to enhance the sensitivity of these devices by matching the resonant frequencies of the carrier and response modes. With these frequencies accurately matched the amplitude of the response mode vibration is amplified by the mechanical quality factor, Q, of the structure. This inevitably makes the construction tolerances more stringent. In practice, it may be necessary to fine tune the balance of the vibrating structure or resonator by adding or removing material at appropriate points. This adjusts the stiffness or mass parameters for the modes and thus differentially shifts the mode frequencies. Where these frequencies are not matched the Q amplification does not occur and the pick-offs must be made sufficiently sensitive to provide adequate gyro performance.
Known vibrating structure gyros based on rings, cylinders or hemispherical shells generally all use a Cos 2.theta. vibration mode. For a perfectly symmetric resonator in the form of a ring two degenerate Cos 2.theta. modes will exist at a mutual angle of 45.degree.. These are shown schematically in the accompanying FIGS. 2A and 2B. One of these modes is excited as the carrier mode as shown in FIG. 2A. For this mechanisation all of the vibration occurs in the plane of the ring. When the structure is rotated about the axis normal to the plane of the ring (z-axis) Coriolis forces couple energy into the response mode as shown in FIG. 2B. This can be understood with reference to FIG. 3. The resonator structure is actually in motion both radially and tangentially. Usually, only radial motion is detected and thus only this motion is considered in FIG. 3 which shows the Coriolis forces acting on a substantially ring-shaped vibrating structure at the anti-nodal points of a Cos 2.theta. carrier mode vibration when a rate is applied around the axis normal to the plane of the ring. The velocity vectors (v) at the points of maximum radial motion are marked. The diagram shows the extremes of deformation of the resonator vibrating structure 4 from its rest position (5) during the course of a vibration cycle. With no applied rate there will be no response mode motion. When the device is rotated about the z-axis the points of maximum radial motion experience Coriolis forces (F.sub.c) as shown. The combination of these forces around the ring sets the degenerate Cos 2.theta. response mode into oscillation. The resulting amplitude of motion is proportional to the rotation rate.
As with the tuning fork structure, enhanced sensitivity may be obtained if the carrier and response mode frequencies are accurately balanced. Choosing a material with radially isotropic properties is of great benefit in achieving this balance. Additional post manufacture fine tuning may still be required to achieve the desired accuracy, however.
In both the commercial and military fields there are numerous applications for inertial sensing units which require two or three axes of rate sensing. This may conventionally be achieved by mounting two or three single axis gyros in the required configuration. A sensor with inherent multi-axis rate sensing capability would be of great benefit for this and such a device would offer a reduction in size, complexity, component count and assembly time with consequent cost reduction.
Devices capable of two axis rate sensing are known. The Vibrating Wheel Gyro (VWOG) design developed at Draper Laboratory, Cambridge, Mass., USA is an example of such a device. This design consists of a ring structure centrally supported by four compliant beams. The resonant carrier mode is a pendulous rotary motion of the ring. Rotations around the x or y axes in the plane of the ring will set the ring into a rocking motion about the input rotation axis. This motion is detected capacitively by plates located under the ring. Such a known device is only capable of operating for two axes rate sensing.